core: implement (*Transaction).GetSignedPart()

Marshalling it and taking all but last byte violates incapsulation
and is just wrong in case transaction already contains any witnesses.

pkg/core/transaction/transaction.go

@@ -197,6 +197,16 @@ func (t Transaction) GroupOutputByAssetID() map[util.Uint256][]*Output {
 	return m
 }
 
+// GetSignedPart returns a part of the transaction which must be signed.
+func (t *Transaction) GetSignedPart() []byte {
+	buf := io.NewBufBinWriter()
+	t.encodeHashableFields(buf.BinWriter)
+	if buf.Err != nil {
+		return nil
+	}
+	return buf.Bytes()
+}
+
 // Bytes converts the transaction to []byte
 func (t *Transaction) Bytes() []byte {
 	buf := io.NewBufBinWriter()

pkg/rpc/request/txBuilder.go

@@ -94,16 +94,7 @@ func SignTx(tx *transaction.Transaction, wif *keys.WIF) error {
 
 // GetInvocationScript returns NEO VM script containing transaction signature.
 func GetInvocationScript(tx *transaction.Transaction, wif *keys.WIF) ([]byte, error) {
-	var (
-		buf       = io.NewBufBinWriter()
-		signature []byte
-	)
-	tx.EncodeBinary(buf.BinWriter)
-	if buf.Err != nil {
-		return nil, errs.Wrap(buf.Err, "Failed to encode transaction to binary")
-	}
-	data := buf.Bytes()
-	signature = wif.PrivateKey.Sign(data[:(len(data) - 1)])
+	signature := wif.PrivateKey.Sign(tx.GetSignedPart())
 	return append([]byte{byte(opcode.PUSHBYTES64)}, signature...), nil
 }